mysql选择与leftjoin匹配的两个匹配行的最新时间戳

Question

I have two tables in MySQL.

The logs table has columns: time, user_id, event_type

The users table has columns: email, id

Every time a user logs in to the system that generates the logs file, the event_type column value is logged with the value '/dashboard'.

The select statement here will show my every time they logged in...

SELECT users.email, logs.time 
FROM users 
LEFT JOIN logs ON users.id=logs.user_id 
WHERE logs.event_type LIKE '/dashboard' 
GROUP BY email, time;

...but I want to return the result set that shows the email address and timestamp for only the first time each user logged in.

Answer 1

You can use min for that:

SELECT users.email, min(logs.time)
FROM users 
    INNER JOIN logs ON users.id=logs.user_id 
WHERE logs.event_type LIKE '/dashboard' 
GROUP BY email;

Also no need for an outer join -- you are negating that in your where criteria. If you want to keep the outer join and return users that might not have matching records in the logs table, then move the criteria to the on :

SELECT users.email, min(logs.time)
FROM users 
    LEFT JOIN logs ON users.id=logs.user_id 
        AND logs.event_type LIKE '/dashboard' 
GROUP BY email;